You can always get into any computer to which you have physical access. The only real protection is a locked room (or maybe a big dog..
In Linux (with GRUB bootloader):
when the GRUB menu appears, hit any key to stop the count
<if you have a graphical GRUB screen, hit escape to go to text mode>
select the OS to boot, and hit "e" for edit
select the kernel line, and "e"
add the word "single" (no quotes) at the end of the line
"enter" and then "b" to boot
You will now be in a terminal with no password required.